In the fabrication of integrated circuits, patterned metal layers are employed to provide electrical interconnection between various devices formed within the substrate. Often times, two or more metalization layers are employed as part of the interconnection scheme. Conventionally, the precess of forming metal interconnects begins by forming an insulative layer over the substrate. This layer (usually silicon dioxide) insulates the substrate from the subsequently deposited metalization layer.
Next, utilizing traditional photolithographic steps, contact openings are etched through this insulative layer to expose the underlying silicon. After a first metalization layer has been deposited and patterned, the entire substrate is covered with another insulative layer--usually silicon dioxide. Small openings, commonly referred to as "via" openings, are then etched in the second insulative layer. When a second metalization layer is subsequently deposited, these via openings establish the electrical connection points between the first and second metalization layers.
Typically, the metal layers which make up the electrical interconnects are deposited by the well-known process of sputtering. Sputtering is performed by impinging high energy ions onto a metallic target. The ions are accelerated in such a way that they bombard the target, causing metallic atoms to be released from the surface of the target. These metallic atoms are directed onto the surface of the silicon substrate where they are deposited, forming the multiple metalization layers described above. Acceleration of the ions is normally achieved by the use of high electric and magnetic fields--the magnetic field being generated by a magnetron.
A magnetron is a device consisting of a group of magnets arranged in a definite configuration. The magnetic array is typically mounted onto the back side of the metallic target so as to create a large magnetic field across the back plane of the target. The flux lines emanating from this field also extend well beyond the target's front surface.
One problem which arises in sputtering devices which use a magnetron is that the strength of the magnetic field varies non-linearly across the target surface. The result is that accelerating ions impinge the target more frequently in areas where the magnetic field is the strongest. This causes the target to erode unevenly which results in a significantly shorter target life. As is appreciated by practitioners in the art, a spatially fluctuating magnetic field is also a source of nonuniformity in the deposited metal layer. This latter effect manifests itself in a phenomena known as step coverage asymmetry. Asymmetric step coverage of the deposited metal film over via contacts results in measurement (i.e., scaling) errors during photolithographic processing. This problem is especially serious for fabrication processes which rely upon step-and-repeat exposure techniques.
Past attempts to relieve these problems have focused on rotating the magnetron about an axis centered about the target. However, rotation of the magnetron has proven ineffective in eliminating the problem of nonuniform target erosion. Instead, magnetron rotation produces an erosion pattern shaped like concentric rings across the surface of the target, each ring corresponding to the varying strength, or peaks of the magnetic field. Moreover, magnetron rotation has been ineffective at eliminating the problem of via step coverage asymmetry.
Therefore, what is needed is a means of improving the uniformity and step coverage symmetry of the deposited metalization layer while avoiding nonuniform erosion of the target.